1. Technical Field
Exemplary embodiments of the invention relate to a system and method for determining a state of a signal lamp.
2. Discussion of Art
To upgrade to interoperable Positive Train Control (PTC), railroads can implement wayside technologies that enable wireless communications of signal aspect, switch position, and hazard detector status information to the locomotive (or other rail vehicle) and/or a central control facility. In some cases, wayside signaling locations may be controlled by relays or other equipment that cannot be easily upgraded to obtain the signal status information via software. In such cases, information such as switch or signal status can be determined by installing sensors on the wires to the track switches or signal lamps.
Traditional techniques for determining the state of a signal lamp utilize configurable thresholds or a single set of thresholds applicable for a single load type. The thresholds for determining whether these signal lamps are on or off can vary based on the type of device and the accompanying circuitry used in a particular implementation. To further complicate matters, circuits used to flash the signal lamps may not turn the signal lamps fully off during the off portions of a flashing cycle. There are a couple of reasons for this. One is that it reduces the amount of thermal shock to incandescent filaments during flashing, which can prolong their life. The other is that it allows for simpler relay logic to be designed when using special relays that do not respond to the signal when it flashes due to the non-zero current that flows during the off portion of the flash. Therefore, if only off/on thresholds are considered, a set of thresholds may be applicable only to a particular load type and flashing circuit configuration. Due to this, these thresholds may be implemented as a user configurable parameter to take into account the various load types and other variables. While this provides flexibility, it may burden the end user to make sure the thresholds are properly configured not only during initial setup and installation, but anytime the signal lamps are replaced. This may result in a test to determine if the thresholds are properly configured for a given load type. For example, if a signal maintainer replaces an 18 W bulb that has burned out with a 25 W bulb, the thresholds must be reconfigured and a test performed to verify the thresholds properly detect the state of the new load type.